See Blue Annals, p. 849-855 (Skor chung & Skor Ni ru pa).
A quote from a book in progress: "Nirūpa is one of the most intriguing figures in Tibet's history, even if one of its lesser known lights. Although he is credited with spearheading the first Mahāmudrā teaching lineage in Tibet, only one sustained passage about him is known to me (he may have been rather neglected because his lineage comes, by way of an Indian named Karopa, from Maitripa, whose Mahāmudrā teachings have been criticized by some). Nirūpa was two persons in one. He was born in 1062 (some place the date sixty years later), and was known at first as Korchung (Skor chung). As a young man, he visited Nepal where, on his deathbed, his physical form was taken over by an Indian disciple of Karopa by the name of Nirūpa. Subsequently known as Kor Nirūpa, back in Tibet, he sometimes dressed in Tibetan and sometimes in Indian garb."
Lewis, Chronology, p. 155. "Dam pa sKor, originally a musk trader, visits Nepal with famous Tibetan teachers, studies Kriyā and Yoga tantras there; gets Mahāmudrā yoginī initiation and practices with Nepalese mudrā [Blue Annals, pp. 849-853]."
The following passage from Khro lo chen pos mdzad pa'i dpag bsam 'khri shing (I have a PDF scan of this manuscript), fol. 11v, is possibly about him (it happens in the first part of Khro Lo's 19th year, which means about 1190, rather late for Skor Nirupa, actually): de nas lo bcu dgu pa'i bstod pa bla ma skor lo tstsha ba bya ba yul nyang ro skar gyi shab bros pa yin / rgya gar du lo sum bcu nye ba cig bzhugs / sgra tshad sngags dang gsum la mkhas par sbyangs pa cig yod de / nang pa'o nang rol na paṇḍi ta rnams dang gsung gleng mdzad pa la / phyi rol nas rgya gar ba'i grwa pa rnams kyi skad gnyan yang bye brag [12r] mi shes pa yin skad / mkhas pa de bod du gser 'tshol du byon pa la / rje btsun khro phu ba dang zhal nas zhabs thog mdzad / skar lo rje btsun de la dad pa thob nas dags po ba'i lugs kyi ngo sprod zhus ... [Khro Lo is inspired to become a Lotsawa, so he studies with Zhang Dge ba, aka Zhang Lo].
